1. Field of the Invention
This invention relates generally to a method for the catalytic purification by hydrogenation of crude isophthalic acid dissolved in a polar solvent, and more particularly concerns the use in such purification of a catalyst comprising either a single Group VIII noble metal component other than a palladium-containing component or a plurality of Group VIII noble metal components comprising at least two of palladium-, platinium, rhodium-, ruthenium-, osmium, and iridium-containing components, and the modulation of the solution hydrogen concentration during hydrogenation.
2. Discussion of the Prior Art
Polymer grade or "purified" isophthalic acid is one of the starting materials which are employed in the manufacture of unsaturated polyesters. Purified isophthalic acid is derived from relatively less pure, technical grade or "crude" isophthalic acid by purification of the latter utilizing hydrogen and a noble metal catalyst, of the type described in Meyer, U.S. Pat. No. 3,584,039 or Stech et al., U.S. Pat. No. 4,405,809 for the purification of crude terephthalic acid. In the purification process, the crude isophthalic acid is dissolved in water at an elevated temperature, and the resulting solution is hydrogenated, preferably in the presence of a hydrogenation catalyst containing a noble metal, typically palladium, on a carbon support, as described in Pohlmann, U.S. Pat. No. 3,726,915 for the purification of crude terephthalic acid. This hydrogenation step converts the various color bodies present in the crude isophthalic acid to colorless products.
However, even after the aforesaid purification, the purified isophthalic acid product contains color bodies. Therefore, it is highly desirable to reduce the concentration of such color bodies that remain in purified isophthalic acid. The color level of purified isophthalic acid product is generally measured directly either by measuring the optical density of solutions of purified isophthalic acid or the b*-value of the solid purified isophthalic acid itself. The optical density of purified isophthalic acid is measured as the absorbance of light at 340 and 400 nanometers (nm) in its basic solution in a solvent such as sodium hydroxide or ammonium hydroxide.
The measurement of the b*-value of a solid on the Hunter Color Scale is described in Hunter, The Measurement of Appearance, Chapter 8, pp. 102-132, John Wiley & Sons, N.Y., N.Y. (1975), and in Wyszecki et al., Color Science, Concepts and Methods, Quantitative Data and Formulae, 2d Ed., pp. 166-168, John Wiley & Sons, N.Y., N.Y. (1982).
More specifically, the b*-value of purified isophthalic acid can be determined using, for example, a Diano Match Scan Spectrophotometer as follows. Purified isophthalic acid is pressed into a pellet having a thickness of about 0.25 inch and a diameter of about 1 inch. The pellet is then irradiated with white light that has been UV-filtered. The spectrum of the visible light reflected from the sample is determined and tristimulus values (X, Y, and Z) are computed using the CIE Standard Observer functions. Using the weighted-ordinate method, tristimulus values are obtained from the following equations: ##EQU1## where R.sub..lambda. is the percent reflectance of the object at wavelength .lambda. and x.lambda., y.lambda., and z.lambda. are the Standard Observer functions at wavelength .lambda. for CIE Illuminant D65. The tristimulus values, X, Y and Z, identify the color of the object in terms of the mixture of the primary lights that match it visually. Tristimulus values, however, are of limited use as color specifications, because they do not correlate with visually meaningful attributes of color appearance and are not uniform in the spacing of colors as related to visual difference. As a result, "Uniform Color Scales" (UCS) have been adopted which use simple equations to approximate visual response. The UCS scale used by the Diano instrument is the CIE 1976 L*a*b* formula which converts tristimulus values to L*, a*, and b* values as shown below: EQU L*=25(100Y/Y.sub.o).sup.1/3 -16
a*=500[(X/X.sub.o).sup.1/3 -(Y/Y.sub.o).sup.1/3 ]
b*=200[(Y/Y.sub.o).sup.1/3 -(Z/Z.sub.o).sup.1/3 ]
The L*-value is a measure of the luminosity or whiteness of an object where L*=100 is pure white, L*=0 is black, and in between is gray. The L*-value is strictly a function of the tristimulus Y-value. The b*-value is measure of the yellowness-blueness attribute where positive b*-values represent yellow appearance and negative b*-values represent blue appearance. The b*-value is a function of both tristimulus values Y and Z.
However, the variable nature of the impure isophthalic acid feedstock makes process control and thus quality assurance difficult and costly. For example, the commercial specifications for purified isophthalic acid may include a predetermined b*-value and/or optical density, and deviations from the predetermined b*-value and/or optical density that are either above or below the predetermined b*-value and/or optical density are undesirable. For this reason, it is highly desirable, and it is an object of the invention, to effect hydrogenation of an impure isophthalic acid solution under conditions that optimize control of the reduction of colored compounds. The present invention provides a convenient method for accomplishing this objective.
The following are believed to be relevant prior art disclosures:
Puskas et al., U.S. Pat. Nos. 4,394,299 and 4,467,110 disclose the use of a combination noble metal catalyst, for example, a palladium/rhodium catalyst on a porous carbonaceous surface, for purification of aqueous terephthalic acid solutions. These two patents also show the use of a rhodium-on-carbon catalyst under reducing conditions and review various heretofore known methods of preparing a Group VIII metal catalyst having activity and selectivity suitable for the purification of terephthalic acid by hydrogenating its principal impurity, 4-carboxybenzaldehyde, to p-toluic acid.
Schroeder et al. copending application Ser. No. 344,657, discloses a method for the purification of crude terephthalic acid, in which an aqueous solution of the crude terephthalic acid, at a temperature of from about 100.degree. C. to about 350.degree. C. and at a pressure sufficient to maintain the solution substantially in the liquid phase, is passed through a particulate catalyst bed and in the presence of hydrogen. The particulate catalyst bed is a layered bed that includes a first catalyst layer supported on an active carbon carrier and containing a metal of Group VIII of the Period Table of Elements having a single electron in its outermost orbital when in the ground state, and a lower catalyst layer containing palladium supported on an active carbon carrier. The solution is passed first through said first layer and thereafter through the second layer. Thereafter the hydrogenated aqueous solution is cooled to effect separation of the resulting purified terephthalic acid from the solution by crystallization.
Packer et al., U.S. Pat. No. 4,626,598, discloses a continuous method for producing purified terephthalic by the catalytic hydrogenation of crude terephthalic acid solution at 530-550.degree. F. in a polar solvent in a reactor in which the solution hydrogen concentration is modulated during the hydrogenation so as to maintain a predetermined color scale b*-value in the resulting purified terephthalic acid. More particularly, the process involves recovering purified terephthalic acid from its solution, which had been hydrogenated at a given reactor hydrogen partial pressure measuring the color scale b*-value of the recovered purified terephthalic acid, and then based on the measured color scale b*-value, adjusting the reactor hydrogen partial pressure from the given reactor hydrogen partial pressure to provide purified terephthalic acid having a predetermined color scale b*-value. The hydrogen partial pressure within the reactor is maintained within the range of from about 10 to about 100 psi. A 0.1-unit change in the color scale b*-value can be effected by an adjustment of the hydrogen partial pressure from about 5 psi to about 60 psi. In the alternative, the optical density at 340 nanometers of a solution of the purified terephthalic acid which had been produced at a given reactor hydrogen partial pressure is measured and then based on the measured optical density, the reactor hydrogen partial pressure is adjusted from the given reactor hydrogen partial pressure to provide a purified terephthalic acid solution having a purified optical density. For 0.1-unit change in the optical density can be effected by an adjustment of the hydrogen partial pressure in the range of from about 2.5 psi to about 25 psi. For either of these alternatives when the hydrogenation is performed in a hydraulically full reactor, instead of adjusting the reactor hydrogen partial pressure from a given reactor hydrogen partial pressure, the gaseous hydrogen flow rate to the reactor can be adjusted from a given gaseous hydrogen flow rate to the reactor. A 0.1-unit change in the optical density can be effected by an adjustment in the solution hydrogen concentration in the range of from about 0.015 to about 0.15 cubic centimeters at 1 atmosphere absolute and 0.degree. C. per gram. It is especially important to note that Packer et al. disclose that the b*-value and/or optical density of purified terephthalic acid are decreased by increasing the solution hydrogen concentration.
Schroeder et al. copending application Ser. No. 257,511, filed Oct. 13, 1988 discloses a method for the purification of crude isophthalic acid in which aqueous solution of said crude isophthalic acid, at a temperature of from about 100.degree. C. to about 300.degree. C. and at a pressure that is sufficient to maintain the solution substantially in the liquid phase, is passed through a particulate catalyst bed and in the presence of hydrogen. The particulate catalyst bed comprises Group VIII noble metal-containing components comprising at least two of palladium-, platinum-, rhodium-, ruthenium-, osmium- and iridium-containing components, supported on active carbon carrier particles. Thereafter the resulting hydrogenated aqueous solution is cooled to effect separation of the resulting purified isophthalic acid from the solution by crystallization.